1. Field of the invention
The present invention is generally concerned with optical components used to correct sight.
It is more particularly directed to intra-ocular implants although it is also applicable to contact lenses.
2. Description of the prior art
As is well known, an intra-ocular implant is designed to replace a defective crystalline lens.
An intra-ocular implant of this kind usually has no ability to accommodate.
This is the case, for example with the implant which is the subject of the U.S. Pat. No. 4,504,982.
In this American patent the central part of the front surface of the intra-ocular implant concerned forms an aspherical surface of revolution with a meridian section satisfying the equation: ##EQU1## in which R, K, A2, A3, A4 and A5 are numerical parameters. However, considered in its entirety this intra-ocular implant is of constant power, the numerical parameters in question simply being chosen so that most of its longitudinal spherical aberration is corrected.
Similarly, the intra-ocular implant which is the subject of the U.S. Pat. No. 4,769,033 has no ability to accommodate in that, being a bifocal lens, it makes no provision for intermediate vision between far vision and near vision.
The obvious disadvantage of intra-ocular implants of this kind with no ability to accommodate is that they are not inherently satisfactory for all kinds of vision and so require the occasional wearing of eyeglasses, especially in the case of a constant power intra-ocular implant.
An accommodating intra-ocular implant is described in the U.S. Pat. No. 4,710,193, however.
This intra-ocular implant is a diffractive device, however, and in practise causes non-negligible chromatic aberration.
An object of the present invention is an accommodating optical device, in particular in intraocular implant, which is advantageously free of this disadvantage.
Until now intra-ocular implant design calculations have essentially been based on the single power that the implant is required to have, with the implant isolated in air, and without reference to the longitudinal spherical aberration to which the implant gives rise in the eye. The invention departs from this conventional thinking by regarding an intra-ocular implant of this kind, once fitted, as forming one component of an optical system whose other components are part of the eye concerned and by conferring upon the intra-ocular implant the surface shapes required to produce a given longitudinal spherical aberration within this optical system, as estimated by calculation.
The problem arises that the component parts of an eye and therefore their characteristics vary from one person to another.
The invention also proposes to use a particular eye model as a reference standard.
This is preferably the eye model described by R. NAVARRO et al in an article entitled "Accommodation-dependent model of the human eye with aspherics" in J. Opt. Soc. Am. A, vol. 2, No. 8, Aug. 1985 which is incorporated in the application by reference.
A different eye model could equally well be chosen instead of this one, however.